For example, I will update the firmware on the ASUS DSL-N12U modem revision B1, from version 1.0.5.1 to 1.0.5.2.
I will describe the steps in steps:
1) Download the new firmware from the official site
https://www.asus.com/support/Product-Search/?keyword=dsl-n12u
2) Open the router settings by typing the address http://192.168.1.1 in the browser (maybe 192.168.0.1) and enter the standard login – admin, the password – admin
3) In the interface that opens at the bottom left, open the Administration menu, select the Firmware Update tab, click Browse in the window that opens and select the firmware that you previously downloaded and unpacked from the archive, and click Submit to start the update process. We are waiting for the completion of the process about 2-5 minutes. You cannot turn off the power of the device at the time of the update.
Done.